The present invention relates to the field of communications by radio. More particularly, it relates to time and frequency acquisition and tracking in radiocommunications systems. It applies in particular to cellular radiocommunications systems, to cellular radiocommunications systems using satellites, or to other wireless radiocommunications systems.
In such radiocommunications systems, problems arise concerning synchronization of the various terminals or stations which are in communication in the system, so as to make it possible for the transmitted signals to be demodulated correctly. A user station which generally does not have accurate clock generator means, must acquire time and frequency synchronization with a transmitter station in order to be able to demodulate the signals it receives from said transmitter station. It is also essential for a user station to be able to perform tracking in time and in frequency, i.e. to maintain time and frequency synchronization throughout a call so as to enable it to continue correctly demodulating the signals it receives.
This problem arises in particular in systems that make use of spread spectrum modulation in which demodulation is possible only when synchronization is acquired. It also arises in low earth orbit (LEO) satellite systems where satellites provide geographical coverage that varies for a given user station.
Physical channels are defined in such radiocommunications systems enabling information to be transmitted and typically corresponding to a carrier frequency; such physical channels are subdivided into logical channels, e.g. by time division or by code division, so as to provide multiple access on a single physical channel. Time division multiple access (TDMA) and code division multiple access (CDMA) are known per se.
Communicating with user stations in a radiocommunications system also requires system information to be broadcast to the stations by using a logical channel associated with a physical channel. By way of example, the system information can comprise the name or the number of the base station (xe2x80x9cgatewayxe2x80x9d), or the geographical location in cell terms. The term xe2x80x9cbroadcastxe2x80x9d is used below to designate transmitting such system information, and the term xe2x80x9ctrafficxe2x80x9d is used to designate the content of the messages interchanged with users. The data rate for system information is usually quite low compared with the data rate for traffic, and is typically a few kilobits per second.
One of the constraints in such a radiocommunications system lies in reaching all of the user stations in the coverage zone, including those which are least favored from the propagation point of view. This constraint is in addition to constraints associated with limitations on spectrum and power resources specific to each type of system.
To perform these functions of acquiring and tracking synchronization and of broadcasting system information, proposals have been made to use a plurality of physical channels, for example a high power unmodulated pilot channel and another physical channel for broadcasting the information. The unmodulated pilot channel makes it easy for the user station to latch on to the frequency of the channel and to achieve time synchronization. Frequency synchronization is acquired by conventional methods of the fast Fourier transform (FFT) type. That method using an unmodulated pilot is implemented in particular in the Qualcomm CDMA radiocommunications system.
That solution suffers from drawbacks. The use of a high power unmodulated channel makes time synchronization simpler, but consumes a large amount of power. In addition, the use of an unmodulated physical channel occupies a plurality of logical resources, and can therefore diminish the capacity of the system. Finally, that solution implies quite complex sequencing of procedures: firstly it is necessary to acquire time synchronization and frequency synchronization on the unmodulated channel, then it is necessary to go to some other logical channel to receive the system information, and finally it is possible to demodulate the traffic channels.
The invention proposes a solution to the problem of acquiring and tracking synchronization in a radiocommunications system. It also proposes a solution to the problem of broadcasting information to user stations.
Unlike known solutions, the invention proposes using a single logical channel, preferably without increasing power, both for the purposes of acquiring and tracking synchronization in time and frequency, and also for broadcasting system information.
It thus makes it possible to make savings in power expenditure, and to limit the amount of resources (physical channels) used solely for synchronization; it also makes it possible to simplify the procedures for acquiring and tracking synchronization.
More precisely, the invention provides a synchronization channel for a radiocommunications system between a plurality of stations, the system having a detection word that is repeated on the channel, and system information that occurs between repetitions of the detection word.
Advantageously, the repetitions of the detection word form a periodic pattern in the channel, the period of the pattern being a submultiple or a multiple of a frame period of the channel.
The detection word can also be repeated periodically, at a period that is a submultiple or a multiple of a frame period of the channel.
In a preferred embodiment of the invention, the system information is transmitted on the channel after being spread.
Advantageously, the spreading is spreading by M-ary modulation using a Hadamard code.
Preferably, prior to being spread, the system information is encoded by an error correcting code.
The invention also provides a radiocommunications system having such a synchronization channel.
Advantageously, the system has a plurality of physical channels, each physical channel having at least one logical channel; the synchronization channel is a logical channel.
Preferably, the logical channels are defined by code division multiple access.